The current turnstile junction industry standard is a topology in which the four (4) branching waveguides are assigned to the lower frequency feed section (typically transmit (Tx) signal) and the high frequency feed section is aligned with the feed horn axis. Therefore, four (4) receive (Rx) band reject filters or other types of filters must be used to separate the Tx and Rx frequency bands.
Such antenna feed systems tend to be large in volume and mass, especially at lower frequencies such as at C-band (between about 3.4 GHz and 6.8 GHz). The filters used in the Tx waveguides tend to have sections small in size thus limiting both peak and average power handling capabilities.
Modern satellite systems typically employ antennas compatible with advanced multi-channel, multi-frequency payloads. The key component of such satellite antenna is the feed system whose design is essential in achieving required payload functionality. There is a multitude of waveguide components that are typically used in the design of an antenna feed system. These include filters, polarizers, orthomode transducers, diplexers and power combiners or splitters. Depending on the components selection and the quality of their design a particular level of the feed RF (Radio Frequency) performance can be achieved. For wide-band multi-frequency, dual linear or dual circular polarization, typically orthomode junctions (OMJ) based feeds are employed.
An OMJ can be constructed in two design options. In the more common version the axial section of the feed (sharing the symmetry axis with the feed horn) includes the high frequency feed section, typically operating in the receive frequency band. The less common OMJ option features a reversed topology where the axial section of the feed is assigned to the lower frequency band (typically operating in the transmit frequency band).
The isolation between transmitting and receiving frequency bands is accomplished by connecting filters in the high and low frequency signal paths. One of the most critical performance requirements at the feed component level is the signal return loss (too much return loss leads to too much power dissipated into the different antenna components, and increased unacceptable operating temperatures, especially at high Tx power signals, such as 100 Watts and more). In antenna systems operating in dual circular polarization the current requirement for Tx-Tx or Rx-Rx ports isolation is 25-30 dB (decibels). Since in the circularly polarized feed the port isolation is driven by the cascaded return loss of the feed chain components, the task of designing a feed arrangement that can meet such most stringent return loss requirements has become a critical issue in space antenna engineering.
In U.S. Patent Application Publication No. 2004/0140864A1 to Chen et al., published on Jul. 22, 2004, a prior art feed does not employ the features of the present invention listed hereinbelow, hence, a design that would not be able to meet the same level of performance as of the present invention. In particular, Chen et al. teach a common and low frequency path that is implemented in a circular waveguide. Furthermore. Chen et al. high frequency reject filter is a periodic structure that can perform well in terms of providing frequency isolation but it compromises the return loss of the OMJ-filter cascade. Moreover, Chen et al. show relatively long stepped waveguide section implemented in the four high frequency paths.
Accordingly, there is a need for an improved orthomode junction feed assembly with associated filters.